1. Field of the Invention
The present invention relates to a cutting-off method of an automatic lathe, and to an automatic lathe, in which a headstock and a rear headstock are movably provided, and more particularly, in which, during a cutting-off, the headstock may rearwardly move to a working start position by releasing the chucking of a bar material by a main spindle, without requiring any complicated structure, thereby shortening the idle time to eventually improve the workability.
2. Description of the Related Art
One example of a headstock-moving type of automatic lathe is designed as follows. There is a main spindle which holds and rotates a bar material, and the main spindle is rotatably secured to a headstock. The headstock is designed to be movable in an axial direction of the bar material (a Z-axial direction). A guide bush is provided in the front of the headstock, and a tool post having a cutting tool is provided in the vicinity of the guide bush, so that the tool post may move in an X-axial direction perpendicular to the Z-axial direction.
Accordingly, the bar material is held by the main spindle as well as by the guide bush which supports an end of the bar material. The bar material is then worked by the cutting tool mounted on the tool post through movements, for example, of the headstock in the Z-axial direction and of the tool post in the X-axial direction.
When a predetermined cutting operation is completed, the bar material is cut off. The main spindle holds and rotatably drives the bar material during the cutting-off, and when the cutting-off is completed, the main spindle releases the bar material, and the headstock rearwardly moves away from the guide bush. When the headstock returns to a working start position, the bar material is held again by the main spindle to allow to proceed to the next cutting operation. Thus the operation cycle is repeated.
The thus described headstock-moving type of automatic lathe may be provided with a rear headstock. A sub spindle of the rear headstock holds the workpiece (made from the bar material), which has been cut off after completion of the front working, so that the working on the rear of the workpiece may separately be carried out by a cutting tool mounted on the tool post.
The cutting-off of the headstock-moving type of automatic lathe having a rear headstock is described as follows. When the cutting operation on the front side of the bar material is finished, the top end of the bar material is held by a sub spindle of the rear headstock. In such a state, the bar material is held at both sides, namely by the main spindle and by the sub spindle. At that time, both the main spindle and the sub spindle rotatively drives the bar material so that a cut-off tool mounted on the tool post may cut the bar material through movement of the tool post.
After completion of the cutting-off, the headstock releases the bar material and rearwardly moves away from the guide bush until reaching the working start position. Then the bar material is held again by the main spindle to proceed to the next front working. On the other hand, the workpiece cut off the bar material is held by the sub spindle to allow a rear working on the rear of the workpiece.
In the thus described structure, the headstock will have to keep holding of the bar material in the vicinity of the guide bush until completion of the cutting-off, and after completion of the cutting-off, the headstock is eventually allowed to return to the working start position. However, in such a structure, it requires much time to start the next front working, thereby the idle time is lengthened, which results in poor workability.
To overcome the above problem, there is provided an automatic lathe disclosed in a Japanese Patent No. 2566570, of which structure is discussed with reference to FIG. 14 as a prior art.
FIG. 14 is a sectional view showing a partial structure of an automatic lathe according to the Japanese Patent No. 2566570. There is a headstock 201 which rotatably holds a main spindle 202. A tool post 203 is provided in the front of the headstock 201. A guide bush housing 205 is fixed on the tool post 203. A bearing 207 is provided on the inner periphery of the guide bush housing 205, so that a pivot 209 is rotatably supported via the bearing 207. A timing pulley 211 is secured to the pivot 209 so that a timing belt 213 is wound between the timing pulley 211 and another unillustrated timing pulley.
Accordingly, when the other unillustrated timing pulley is rotatively driven by an unillustrated driving motor, the timing belt 213 and the timing pulley 211 are driven, thereby the pivot 209 is rotatively driven.
There is a guide bush 215 inside the pivot 209, and an adjust screw 217 is engaged with and connected to the rear end of the guide bush 215. Further, there are levers 221 mounted on the rear end of the pivot 209 via pins 219. One end of the lever 221 is in contact with a flange 217a of the adjust screw 217, and the other end thereof is protrusively provided toward the outer periphery of the pivot 209.
There is a bobbin 223 provided at the rear end of the pivot 209 to be in slidable contact with the outer periphery of the pivot 209. The other end of the lever 221 as above discussed is in contact with a taper surface 223a provided on an inner periphery of the bobbin 223.
The bobbin 223 is sandwiched and supported by an opening/shutting lever 225 swingably mounted on the tool post 203. The opening/shutting lever 225 is further connected to a piston rod 229 of a hydraulic cylinder mechanism 227 mounted on the tool post 203.
According to above structure, for example, when the piston rod 229 of the hydraulic cylinder mechanism 227 is driven in the protrusive direction, the opening/shutting lever 225 rotates in the counterclockwise direction, thereby the bobbin 223 moves toward the right hand of FIG. 14. Thus the adjust screw 217 and the guide bush 215 move toward the left hand of FIG. 14 via the levers 221. Accordingly, there is formed a proper size of space between the guide bush 215 and a bar material 231.
To the contrary, when the piston rod 229 of the hydraulic cylinder mechanism 227 is driven in the withdrawal direction, the opening/shutting lever 225 rotates in the clockwise direction, thereby the bobbin 223 moves toward the left hand of FIG. 14. Thus the adjust screw 217 and the guide bush 215 move toward the right hand of FIG. 14 via the levers 221. Accordingly, the guide bush 215 holds the bar material 231.
In the thus mentioned automatic lathe, while the guide bush holds and drives the bar material 231 to be rotated, a cut-off tool 233 as shown by virtual (chain double-dashed) line may cut the bar material 231 off. At that time, since the headstock 201 may release the chucking of the bar material 231, the headstock 201 eventually returns to the working start position, thereby the idle time can be shortened, which results in improvement in workability.
However, the above discussed prior art has the following problem.
Firstly, as shown in FIG. 14, the guide bush mechanism including the guide bush 215 as well as the peripheral parts thereof requires the complicated and large-sized structure, due to the rotative driving mechanism provided in the guide bush mechanism, and due to requirement of a clamp mechanism of the guide bush 215 to serve as a collet chuck.
Secondly, during the cutting-off, the automatic lathe in the prior art cannot perform the two-dimensional working such as to form an arc or a taper, since the guide bush 215 holding the bar material 231 cannot move in the Z-axial direction (namely, the lateral directions of FIG. 14).
Further, as illustrated in FIG. 14, the size of space between the guide bush 215 and the bar material 231 is varied during the working cycle. That is, although the space is provided between the guide bush 215 and the bar material 231 during an ordinary working, when the cutting-off is carried out, the bar material 231 is held by the guide bush 215 without space. However, according to the thus described structure, it is difficult to control the size of the space of the guide bush 215, and there will be a case, for example, that the proper size of space cannot be formed during the ordinary working, which may result in poor accuracy of working.